New hybrids of the native eastern hemlock that can survive the invasive insect woolly adelgid are being bred by ARS scientists and their collaborators. Photo of eastern hemlocks courtesy of Paul Bolstad, University of Minnesota, Bugwood.org; inset photo of hemlock woolly adelgid courtesy of Michael Montgomery, Forest Service, Bugwood.org.

Over the past few decades, two hemlocks native to the United States—Tsuga canadensis and Tsuga caroliniana—have been under attack by the small sucking insect Adelges tsugae, also known as the hemlock woolly adelgid (HWA). Originally from Asia, this aphid relative has spread to forests and backyards in 17 eastern states, killing hemlock trees and devastating natural ecosystems.

Under the direction of ARS geneticist Denny Townsend (now retired), the arboretum began a breeding program in the 1990s to develop hemlock hybrids tolerant to HWA. The scientists crossed hemlock species native to the United States with germplasm—collected in Asia—of hemlocks that have shown tolerance to the insect. Now, 10 years later, Olsen and Bentz have developed 140 hemlock hybrids, 108 of which are suitable for testing.

In 2006, Olsen and his colleagues began a multi-year field trial to test each hybrid's degree of tolerance to HWA. Testing more than 170 trees, the researchers artificially infested the hybrids by attaching HWA-infested branches to the hybrids' lower branches and securing them with mesh bags to prevent the insects from escaping. They found the species T. chinensis and its hybrids to be most tolerant to HWA.

The hybrids are appealing not only due to their tolerance, but because they have good vigor and shape. Still, the researchers have several years of testing to complete before they can release these hybrids.

Read more about this research in the November/December 2010 issue of Agricultural Research magazine.